This invention relates to a reversible electric drive with bidirectional back-current brake which is fed by a rectifier supplied from alternating current mains and operated with an electric series-wound motor.
The electric drive according to the present invention can be applied advantageously in an apparatus regulating the volume of energy fed into electric arc furnaces, where the drive serves to move an electrode clamp up and down.
The advantages of electric drive regulators with rectifiers lie in that the voltage fed onto the poles of the motor--and thereby the rotation rate thereof--can be regulated infinitely variably without inserting a mechanical contact. High speed electronic components can be used, therefore the time constant of the system is determined solely by the motor. The use of electronic components also results in lower maintenance requirements, the use of no mechanical components which results in a higher reliability.
The electric drives are generally constructed with thyristor-bridge rectifiers. Concerning the number and placement of the thyristors, there are several known arrangements. One of them consists of four thyristors connected in a full-controlled bridge, in which the motor is placed in the diagonal thereof. In order to reduce the effects of overvoltages, resistors, inductances or RC-networks are inserted in series with the motor. For the sake of lower costs, half-controlled bridges containing two thyristors and two diodes have been constructed, wherein the thyristors are placed either symmetrically or asymmetrically. These bridges have the advantage that the duration of conductive state can be ideal, i.e. 180.degree.. The braking of the motor is generally realized by means of braking resistors connected to the clamps of the motor, which can be switched thereto either when braking, or by means of braking resistors switched in continuously. The reversion is carried out by means of reversing the polarity of the voltage switched on the poles of the motor. For this purpose mechanical or contactless switches are used, or two parallelly connected rectifier bridges. The control signal is always fed into the component yielding a voltage corresponding to the rotation direction.
The complexity of the drive control electronics increases proportionally with the number of thyristors and the complexity of the task, consequently they are expensive and less reliable. On the other hand, in drives applied for arc furnaces the apparatus containing complex electronics can be used only together with auxiliary protective apparatus. Therefore the applied circuits are generally simple in construction. In these arrangements the braking resistor is--in order to abtain a short recovery time--fixed parallelly with the motor. The result is that the consumption is two to three times higher than the driver energy, as the wasted power is equal to or greater than the active power of the motor.